In the recovery of tritium from the exhaust gas of a nuclear fusion reactor, elemental hydrogen isotopes must be separated from other gas components, i.e. impurities, the elemental hydrogen isotopes being further processed for separation of the components protium, deuterium and tritium, and the impurities being sent to waste disposal. However, since the impurities generally contain tritium in chemically bound form, this must be recovered before the impurities can be safely disposed of. The separation of impurities from the exhaust gas and the processing of impurities to recover tritium is accomplished by a fuel processing loop.
The main requirement for the fuel processing loop (FPL) is to receive tritiated hydrogen (Q.sub.2 where Q=H, D or T) streams containing small amounts of impurities such as C.sub.n Q.sub.m, CO, A, CO.sub.2, N.sub.2, NQ.sub.3, O.sub.2, and Q.sub.2 O, to separate the tritiated hydrogen from the impurities, and to send it in the form Q.sub.2 to the isotope separation system for final tritium purification. The remaining tritium-depleted impurities are sent to a waste gas treatment system.
A number of different FPL processes have been proposed:
Pd/Ag Permeation with Catalytic Impurity Decomposition [R.-D. Penzhorn, R. Rodrigues, M. Gugla, K. Gunther, H. Yoshida and S. Konishi, "A Catalytic Plasma Exhaust Purification System", Fusion Technology 14, p. 450, 1988. ]Hydrogen isotope purification by palladium/silver alloy permeators combined with selective catalytic decomposition reaction steps which avoid intermediate conversion of impurities into water.
Catalytic Oxidation with Hot U-Bed Water Decomposition [J. L. Hemmerich, A. Dombra, C. Gordon, E. Groskopfs and A. Konstantellos, "The Impurity Processing Loop for the JET Active Gas Handling Plant, Fusion Technology 14, p. 450, 1988.]All impurities are fully oxidized in a catalytic recombiner, the tritiated water frozen in a cold trap and subsequently decomposed on hot uranium powder. Hydrogen isotopes set free in this reaction are scavenged from the He carrier gas in a cold U-Bed.
Pd/Ag Permeation with Catalytic Oxidation and Electrolysis [S. Konishi, M. Inoue, H. Yoshida, Y. Naruse, H. Sato, K. Muta and Y. Imamura, "Experimental Apparatus for the Fuel Cleanup Process in the Tritium Processing Laboratory", Fusion Technology 14, p. 596, 1988.]Hydrogen isotopes are separated from impurities by a Pd/Ag permeator. All tritiated impurities are oxidized and the tritiated water is electrolysed to form elemental hydrogen isotopes which are removed by a second Pd/Ag permeator.
Hot U-Bed Impurity Decomposition and Cryogenic Adsorption [P. Schira and E. Hutter, "Tritium Cleanup on Hot Uranium Powder", Fusion Technology 14, p. 608, 1988.]The process gas to be purified is passed through hot beds containing fine uranium powder. In this step, the impurity compounds are cracked and the elements 0, C and N are adsorbed as uranium oxides, carbides and nitrides. At temperatures of 500.degree. C. and above, the hydrogen isotopes no longer produce hydrides with uranium and pass through the bed. In a downstream molecular sieve, all remaining impurities and some hydrogen are cryogenically adsorbed at -196.degree. C.
Cyrosorption, Catalytic Oxidation and Electrolysis [A. Ohara, K. Ashibe and S. Kobayashi, "Fuel Purification System for a Tokamak Type Fusion Reactor", Fusion Engineering, Proceedings Volume 1, 12-th Symposium on Fusion Engineering, Oct. 12-16, 1987, Monterey, Calif., p. 743.][E.C. Kerr, J.R. Bartlit, and R. H. Sherman, "Fuel Cleanup System for the Tritium Systems Test Assembly", Proceedings of ANS Topical Meeting, Tritium Technology in Fission, Fusion and Isotopic Application, 1980, p. 115]. Cryogenic adsorption is used to separate impurities other than helium. The separated impurities are then catalytically oxidized to convert NQ.sub.3 and C.sub.n Q.sub.m to Q.sub.2 O, H.sub.2 O and N.sub.2. The water is then electrolysed and sent to the isotope separation system.
In all the above designs, it has been assumed that the FPL must decompose all the C.sub.n Q.sub.m, Q.sub.2 O and NQ.sub.3 impurities into a Q.sub.2 stream, which is then sent to the isotope separation system. No consideration has been given to simply swamping the impurity stream with H.sub.2 and then exchanging the tritium in the impurity compounds with protium (H). This approach has probably not been considered because it would increase the H/T separative duty of the isotope separation system. However, applicants' recent design studies have shown that sizing of the isotope separation system for H/T separation is determined mainly by requirements for waste water detritiation and pellet injector propellant clean-up. The addition of a small additional H.sub.2 /HD/HT stream from the FPL has virtually no impact on the isotope separation system design.
The object of this invention is to provide an improved process for the recovery of tritium based on the aforementioned discovery.